Method of generating a position error signal, method of writing a data track, and method and apparatus for testing a head

ABSTRACT

A method of generating a position error signal for a desired radial position of a read/write head relative to a data track of a disk is disclosed. The track has a plurality of servo bursts defining a plurality of servo nulls for the track, and are positioned such that they are at more than four different radial positions relative to the track, and define a predetermined locus having a known position relationship with the track. The method comprises determining a target null position on the null locus corresponding to the radial position of the head relative to the track; detecting the position of the servo null with the head; determining from the detected servo null position the position error of the head relative to the target null position; and, generating a position error signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase of PCT/GB2007/000402, filed Feb. 6,2007, which in turn claims priority to U.S. provisional application Ser.Nos. 60/771,879, filed Feb. 10, 2006 and 60/817,084, filed Jun. 29,2006, the entire contents of all of which are incorporated herein byreference.

The present invention relates to a method of generating a position errorsignal, a method of writing a data track, and a method and apparatus fortesting a head.

In embodiments, the present invention relates generally to head mediatest apparatus such as are commonly known as “spin-stands” in the art.

Spin stands were first developed in the art as a tool for use duringresearch and development to allow the performance of the variouscomponents of disk drives, for example the heads, disks and channels, tobe evaluated and optimised. It is now common to also use spin stands inthe field of disk drive manufacturing to test each manufacturedread/write head or disk before it is assembled into a disk drive unit.

A typical test apparatus known in the art comprises a motor drivenspindle on which a disk is mounted that can be written to and read bythe head under test, and a support assembly for supporting the headunder test and “flying” the head over the disk when spun. The testapparatus also comprises an arrangement to allow the head to bepositioned over the disk. This typically comprises a coarse positioningdevice, such as an X-Y positioning stage, for positioning the headanywhere over the surface of the disk.

The coarse positioning device is used to position the head generally inthe region of a test track on the disk. A fine positioning device, suchas a piezo-element actuator or similar micro actuator, is typically alsoprovided for fine positioning of the head. The fine positioning deviceis used to find and locate the head over the centre of the track andthen to “microjog” the head over small increments from the track centre.

When conducting a test, the head is first positioned over the centre ofthe track, and the fine positioning device is then used to position thehead at various radial positions on or across the track. Test data iswritten to the track and subsequently read back by the head at thevarious radial offsets of the head. Typically readings must be takenover several revolutions of the disk at each offset to reject noise. Inthis way a series of tests may be conducted, including for exampleso-called bit error rate (BER) bathtubs, track squeeze, track centre,read/write offset, etc.

Usually, the fine positioning device has a highly accurate internalsensor for determining the error between the desired position input tothe fine positioning device (the so-called “commanded position”) and theactual position achieved by the fine positioning device. This error termcan be fed back into a control loop for the fine positioning device inorder to reduce or eliminate any error and achieve a more accuratepositioning. However, despite the fine positioning device having thisfeedback control, there is no way of the system directly determining anyerror that has been introduced between the desired or commanded headoffset and the actual head offset. This means that the positioning ofthe head at the various offsets relative to the track centre iseffectively open loop. In practice, this is undesirable, as noise fromvarious sources and particularly from thermal drift can affect thesystem in a way that the internal sensor of the fine position sensorcannot “see” and which therefore the system cannot compensate for,thereby leading to error in both achieving the desired offset andmaintaining the desired offset.

To address this, it has been suggested in the prior art to use servotracks on the disk to allow the absolute position of the head on thedisk to be determined. Under this scheme, the test track comprises twotypes of data. The first set of data is in the form of servo bursts,which are generally arranged in the form of a sectored servo track.These servo bursts in effect define the position of the test track onthe disk and are used by the head to establish its position relative tothe track in terms of a position error signal (PES), as is generallyknown in the art. The servo bursts may be pre-written to the disk beforethe disk is mounted in the test apparatus. More commonly, the servobursts may be written by a head when the disk is first mounted in thetest apparatus, as part of a disk initialisation process. Regardless ofhow the servo track is written, it is likely that the same servo trackwill be subsequently used for the testing of multiple heads (typicallymany hundreds or thousands of heads).

The second set of data is the test data, which is written interleavedwith the servo bursts of the sectored servo track. This data is writtenanew and read back by each head tested in the apparatus to allow theread/write performance of the individual head to be measured.

The PES derived from the servo bursts is ultimately fed back to a servocontroller and used in reducing any error in the commanded position ofthe head, for example due to thermal drift.

For example, U.S. Pat. No. 6,023,145 and U.S. Pat. No. 6,538,838 bothgenerally disclose arrangements using a servo scheme of this form in ahead media test apparatus.

Under these schemes, the arrangement of servo tracks and generation of aposition error signal from the servo tracks is essentially similar tohow the head positioning system typically operates in a head diskassembly in end use. However as will be explained this system isunsuited for use in a head test apparatus for a number of reasons.

FIG. 1A shows a track 1 having a typical prior art quadrature amplitudemodulated servo burst frame 2. (The track 1, which is usually circularand concentric with the disk, has been shown here to be linear forsimplicity). The servo burst frame 2 has four series of periodicmagnetic transitions known as servo bursts: respectively the A-burst,B-burst, C-burst and D-burst 3 a,b,c,d. Each servo burst 3 a,b,c,d iswritten within a servo frame 2 so as to repeat at half track radialintervals, and each has a different radial offset from the others.Typically there may be about 250 servo frames 2 circumferentially aroundthe track 1. Typically each servo frame 2 around the track 1additionally contains a unique track address (not shown) that can bedecoded digitally to give a coarse position of the servo frame 2.

When a read/write head 4 (shown in FIG. 1A over the track centreline 5)is flown circumferentially across the track 1 and over the servo burstframe 2, each servo burst 3 a,b,c,d is detected by the head in turn andgives rise to a signal having a strength proportional to the degree towhich the head 4 radially overlaps the respective bursts 3 a,b,c,d. FIG.1B shows how a quadrature amplitude modulated (QAM) position errorsignal (PES) is derived from these four signals. Two traces are derived:a first signal 6 derived from the A and B bursts 3 a,b equal to(A−B)/(A+B); and a second signal 7 derived from the C and D bursts 3 c,dequal to (C−D)/(C+D).

As shown in FIG. 1B, the signals 6, 7 provide a voltage reference thatgives a measure of radial position of the head 4 relative to the track 1which is quasi-linear in each quadrant Q1,Q2,Q3,Q4 of the track 1. Aspart of the demodulating process the pair of bursts 3 a,b;3 c,d chosenis the one that gives the most linear signal 6;7 for the quadrant inwhich the head is located (i.e. C and D for Q1, A and B for Q2, etc.).Thus a reasonably quasi-linear PES is generated for the head 4 whicheverquadrant Q1,Q2,Q3,Q4 it is in. Nevertheless, even in each quadrantQ1,Q2,Q3,Q4 there will be some degree of linearity error which willintroduce errors into the system if not corrected for.

In addition this servo scheme is likely to be affected by gain error.This is typically caused by variation in the width of the read elementof the heads being tested. The width of the read element can vary up to20% due to manufacturing tolerances. This variation in width will leadto a slightly different signal generated when detecting a given servoburst with different heads and thus to a gain error. The two traces 6 a,6 b of the first signal shown in FIG. 1B correspond to head readelements of different widths 4 a,4 b as shown in FIG. 1A and show theeffect of gain error (respectively drawn with full and broken lines).

It will be appreciated that there are positions of the head relative tothe track where A=B or C=D, i.e. where the signals detected from theA-burst and B-burst or C-burst and D-burst are identical. At thisposition the gain and linearity of the head do not affect the accuracyof the signal derived from the servo burst. This position where A=B orC=D is named a “servo null” 8. As can be seen from FIG. 1B, for eachquadrant Q1,Q2,Q3,Q4, the signal is generally most linear at the servonull 8 and at positions close to the servo null 8, and least linear atpositions furthest away from the servo null 8.

It should be noted that other servo burst arrangements are possible,including for example amplitude modulated servo bursts and phasemodulated servo bursts. All these suffer from the same basic problemscaused by non-linearity error and gain error.

The linearity and gain errors are less of a problem in an end-use disksystem as the head is normally only required to follow the trackcentreline, which is usually made to coincide with a null position.However as described above, for head testing it is desired to achievehead offsets at many positions across the track and to maintain theseoffsets for several revolutions of the disk. Thus high linearity,accuracy and repeatability of the PES are desirable at all radialpositions across the track during head testing. Also linearity and gainerror in the PES fed back to the closed-loop servo controller adverselyaffects the performance and stability of the controller. Also, becauseof head skew in an end-use disk system, it cannot follow the optimumnull. It therefore takes even longer to characterise the linearity andgain of the head to the servo track prior to use. This is not acceptablein a head/disk test system where a high throughput of parts tested isrequired.

In order to mitigate to some extent the above problems inherent with theservo track arrangement of the prior art, the prior art has suggestedthat each head be characterised to the servo track before testing. Thisis generally done by finding the track centre, and then micro-joggingthe head across the width of the track and calibrating the linearity andgain of the head to avoid errors being introduced into the system.However, this characterisation process is not practical in a productiontest environment because of the time taken to characterise each headunder test to the servo tracks on the test medium.

U.S. Pat. No. 6,023,145 teaches a slightly different servo scheme. Inthis method the head is first moved to a commanded position using theinternal sensor of the micro positioner for reference. At this commandedposition the servo bursts are read by the head, and a position errorterm is determined from readings taken over several revolutions of thedisk and stored in memory. This position error term is then used as areference for the controller, in order to allow the head to be locked inthe desired position even in the presence of thermal drift. However,this method has the disadvantage of requiring many revolutions of thedisk in order to calculate the position error term accurately enough toprovide an adequate reference. This makes the method only suited at bestto use in a research and development laboratory, where it is moreacceptable to keep the head at the same radial position for a largenumber of revolutions of the disk, thereby giving sufficient time forthe position error term to be accurately determined. In contrast, in aproduction environment a full suite of tests are likely to be performedon the head. This calls for the head to be repeatedly micro-joggedacross the track to develop micro track profiles, BER bathtubs, etc.Hence a lengthy calibration process at each micro-jogged position isprohibitively slow. Speed of testing is particularly important in aproduction environment as this directly affects the throughput of unitstested and hence the production costs. For these reasons, the method ofU.S. Pat. No. 6,023,145 is not suited to perform these tests inproduction testing.

In addition to the above described problems with the prior art, thegeneral trend in disk drive technology is for the widths of the headsand tracks to decrease. The above mentioned problems are exacerbated asthis trend continues.

For these reasons, known servo techniques from the field of disk servodesign are of limited assistance in spin stands as they do not providefor very fine positional control across the radial extent of trackneeded when testing the head, and/or require prohibitivelytime-consuming characterisation of each head under test to the servobursts written to the disk.

According to a first aspect of the present invention, there is provideda method of generating a position error signal for a desired radialposition of a read/write head relative to a data track of a disk,wherein the track has a plurality of servo bursts defining a pluralityof servo nulls for the track, the servo nulls being positioned such thatthere are servo nulls at more than four different radial positionsrelative to the track, the servo nulls defining a predetermined locushaving a known position relationship with the track, the locus extendingacross the radial extent of the track, the method comprising: (a)determining a target null position on the null locus corresponding tosaid desired radial position of the head relative to the track inaccordance with said known position relationship; (b) detecting theposition of at least one servo null with the head; (c) determining fromsaid at least one detected servo null position the position error of thehead relative to the target null position; and, (d) generating aposition error signal in accordance with said position error.

The above arrangements provide a position error signal that can be usedin a continuous feedback system of absolute position regardless of wherethe head is positioned and regardless of whether the head is doing atrack-to-track move or a micro-scan across the track for a BERmeasurement or cross track profile. This is accomplished bypre-calculating where the target null will occur prior to the head beingmoved to its intended destination. This target null is the absoluteposition on the disk relative the track and is used as the referencepoint for generating the position error signal. Because the absoluteposition is used, the target within the feedback loop stays the same,leading to the advantage that the PES may be averaged over multiplerevolutions of the disk even when the head is micro-jogging across thetrack during the time when the samples are acquired for averaging. Incontrast, in the system of U.S. Pat. No. 6,023,145 the target within theloop must be reset for every new position to which the head is moved,which requires an extended acquisition time to generate an acceptablereference.

This arrangement also allows the movement of the head to be changed froma step, settle and read method (which is the current method ofconducting head media testing) to a method where the head is moved at aconstant velocity across the written track, eliminating seek and settletimes and further reducing test times.

The above arrangement also has the advantage that the radial positionsof the servo nulls vary across the radial extent of the track owing tothe locus and the track not being aligned with each other. As previouslydescribed, the servo bursts provide the most accurate, repeatable andlinear relationship between the measured head offset and actual headoffset in the region of the servo nulls (herein called the “linear zone”of the servo burst). Hence by having servo nulls with different radialpositions relative to the track, the linear zones surrounding theplurality of servo nulls can be made to extend across a greater radialextent of the track than is the case with prior art servo arrangementswhere the servo nulls and hence the linear zones are radially aligned.

In general, this arrangement has the advantage that a high quality PESvalue can be obtained without having to characterise each individualhead under test to the servo track of the disk as in the prior art. Thissaves a considerable amount of time when testing and therefore, in aproduction environment, reduces costs of manufacturing disk drive units.

In a preferred embodiment, step (b) includes detecting the position ofat least the servo null radially nearest the head. This arrangement hasthe advantage that at any radial position of the head relative to thetrack the head can derive a high quality, repeatable PES based on theservo null that is radially closest to it and that provides the mostaccurate PES.

In a further preferred embodiment, step (b) includes detecting theposition of a plurality of servo nulls and step (c) includesinterpolating between said plurality of null positions in order to findsaid position error. This allows further accuracy to be obtained byinterpolating as required between the discrete servo burst frame samplesto determine a target null position that is not constrained to becoincident with a servo null. Also greater accuracy can be obtained byinterpolating between a large number of servo samples to find theposition error of the head in order to reject noise present in thesamples.

The position error signal may be averaged over successive revolutions ofthe disk. Because a target null position is pre-calculated before thehead moves to a new position and the PES is generated in relation tothis target null, the PES effectively has the same reference point forall positions of the head. Therefore, the PES may be averaged oversuccessive revolutions of the disk before being fed to the servocontroller with no calibration being necessary between moves. Thisprovides further accuracy and noise rejection in the PES signal.

In an embodiment, the desired radial position of the head is selected soas to coincide with a servo null.

In a most preferred embodiment, the track is concentric with the disk,and the plurality of servo nulls are circumferentially spaced and havedifferent radial positions at each circumferential position.

In a preferred embodiment, the position of the servo nulls extend acrossat least a majority of the radial extent of the track. This has theadvantage that whatever the radial position of the head, it is likely tobe close to a servo null and can hence derive a linear PES from at leastthat servo null. In a preferred embodiment, the servo nulls have radialpositions that are evenly spaced. The circumferential positions of theservo nulls may be evenly spaced. This allows a high quality linear PESto be generated from the servo bursts. In a preferred embodiment, theposition of the servo nulls extend successively in a single radialdirection on going round the track.

In the preferred embodiment, the radial positions of the servo nullsvary linearly with circumferential position. This has the advantage ofmaking the interpolation between nulls more simple to implement.

Preferably the servo bursts are formed in respective servo sectors of asectored servo track. This allows the circumferential position of aservo burst to be obtained, for example by providing servo sectoridentifiers that can be read by the head, or by keeping a count of thenumber of servo sectors crossed by the head.

The position of the servo nulls may define at least part of at least onespiral of servo nulls on the disk. This allows a single spiral servotrack to be written to the disk, which can be used to provide a PES atany radial position on the disk. This also provides the advantage thatthe spiral may be written contiguously so that servo tracks can bewritten across the disk with minimal errors being introduced by theeffects of thermal drift. Preferably the pitch of the spiral is thewidth of the track. This provides the greatest number of servo nulls pertrack and hence greater accuracy.

In a further embodiment, the servo null locus is concentric with thedisk. The track may define a sine wave. The sine wave may have awavelength equal to the track circumference. A plurality of servo nullloci may be written to the disk. The track may be written to encompassmore than one servo null locus.

In an embodiment, the disk is initially free of servo bursts, the methodcomprising performing, before step (a), the step of writing said servobursts to the disk.

The servo bursts are preferably quadrature amplitude modulated servobursts. Alternatively, the servo bursts may be amplitude modulated servobursts or phase modulated servo bursts, or any other servo burst havinga null.

In an embodiment, there is provided a method of testing a read/writehead, the method comprising: commanding the head to a desired positionrelative to a track of a disk; generating a position error signal forthe difference in the actual and desired position of the head asdescribed above; controlling the position of the head with a closed loopcontroller arranged to reduce the position error signal substantially tozero; and, testing the head.

According to a second aspect of the present invention, there is provideda method of writing a data track to a disk with a read/write head aspart of a method of testing the head in a head media test apparatus, thedisk having a plurality of servo bursts defining more than four servonulls, the method comprising: writing said data track to coincide withsaid more than four servo nulls, such that the servo nulls define apredetermined locus having a known position relationship with the track,the locus extending across the radial extent of the track.

In a preferred embodiment, the track is concentric with the disk, andthe plurality of servo nulls are circumferentially spaced and havedifferent radial positions at each circumferential position. Theposition of the servo nulls may extend successively in a single radialdirection on going round the track. The radial positions of the servonulls may vary linearly with circumferential position. The position ofthe servo nulls may define at least part of at least one spiral of servonulls on the disk.

In another embodiment, the servo null locus is concentric with the disk.The track may define a sine wave.

In an embodiment, the disk is initially free of servo bursts, the methodcomprising performing, before the step of writing the data track, thestep of writing said servo bursts to the disk.

According to a third aspect of the present invention, there is providedapparatus for testing a read/write head, the apparatus comprising: adisk having a data track, wherein the track has a plurality of servobursts defining a plurality of servo nulls for the track, the servonulls being positioned such that there are servo nulls at more than fourdifferent radial positions relative to the track, the servo nullsdefining a predetermined null locus having a known position relationshipwith the track, the locus extending across the radial extent of thetrack; a positioner for positioning a said head over a radial positionon the disk; a processor arranged to: (a) receive a desired radial headposition relative to the track; (b) determine a target null position onthe null locus corresponding to said desired radial position of the headrelative to the track in accordance with said known positionrelationship; (c) detect the position of at least one servo null withthe head; (d) determine from said at least one detected servo nullposition the position error of the head relative to the target nullposition; and, (e) generate a position error signal in accordance withthe position error; and, a feedback controller arranged to receive saidposition error signal as a feedback input, and to cause said positionerto position said head so as to reduce said position error signalsubstantially to zero.

According to a fourth aspect of the present invention, there is provideda disk, the disk having at least one circular concentric track having aplurality of servo bursts defining a plurality of servo nulls for thetrack, the servo nulls being positioned such that there are servo nullsat more than four different radial positions relative to the track, theservo nulls defining at least part of a spiral on the disk having aknown position relationship with the track, the locus extending acrossthe radial extent of the track.

According to a fifth aspect of the present invention, there is provideda method of testing a read/write head suitable for use in a disk drive,the method comprising: positioning the head over a radial position on aspinning disk medium corresponding to a track on the disk; commandingthe position of the head with a periodic movement signal such that theradial position of the head moves notionally along a periodic path, thehead traversing n wavelengths of the periodic path as the disk rotates mrevolutions, where n and m are integers; reading from the disk using thehead at a plurality of points along the periodic path; and,characterising the head using said readings.

The periodic movement signal has the advantage that the head is able toread data from the track at various radial positions in a single pass (apass being defined as an operation where test data is acquired by theread/write head continuously without stopping to reposition the head ata different offset). In comparison, in the prior art arrangements,multiple passes are needed, one pass being needed for each radial offsetposition of the head. The arrangement of this embodiment has theadvantage that less time is spent positioning the head at desiredoffsets of the disk and more time is spent actually acquiring test data.

The head traverses n wavelengths of the periodic path as the diskrotates m revolutions. This has the advantage that the periodic path“closes” in that the head returns, after m revolutions of the disk, tothe same position relative to the disk surface and follows the same pathrelative to the disk surface. This means that a further set of testreadings can be taken by the read/write head at substantially the samepoints relative to the disk surface. Again, it is unnecessary to stoptaking test readings to reposition the head. This again leads to savingsof time in the testing process.

In one embodiment, n=1 and m=1. This conveniently allows the wavelengthof the periodic path to coincide with a single full revolution of thedisk.

In a preferred embodiment of the method, said readings are taken at thesame positions within each periodic path relative to the disk, themethod comprising averaging said readings for respective points on thedisk over different revolutions of the disk. Averaging the readings fromthe read/write head allows a more accurate set of test readings to beproduced where the effects of noise and/or other factors resulting inspurious readings are mitigated. This allows the read/write head undertest to be characterised with greater accuracy.

In a preferred embodiment, the commanded periodic path is substantiallya sinusoidal path.

In a further preferred embodiment, the commanded position of the headmoves the head over the full radial extent of the track. This allows forexample a full bathtub BER test to be performed in a single pass.

In a preferred embodiment, the method comprises readingoptically-readable marks that rotate with the disk using an opticalreader and determining the rotational position of the disk accordingly;and, calculating the commanded periodic path with reference to saiddetermined rotational position of the disk. This allows preciseco-ordination between the period of the commanded periodic path and therotation of the disk. This allows the commanded periodic path to “close”with good precision.

In a further preferred embodiment, the method comprises providing atleast one error signal in the position of the head to a servocontroller, the servo controller being arranged to control the positionof the head in accordance with said at least one error signal and saidcommanded periodic movement signal. The track may have servo burstsassociated therewith. The method may comprise: detecting the servobursts with the head; demodulating the detected servo bursts todetermine the position error signal of the head; sampling the positionerror signal at positions of said periodic movement signal thatcorrespond to the desired track centre points; and, determining thetrend in the samples over a plurality of samples to provide said atleast one error signal. This allows the position of the head to becontrolled to compensate for noise or drift that has affected thepositioner and caused the actual offset of the head to differ from thecommanded offset of the head. This has the advantage that the bandwidthof the samples is relatively low, allowing the controller function to bemore simply implemented. In addition, the samples are taken (at leastnotionally) from the same position relative to the radial position ofthe track, namely the centre of the track, where the servo nulls arepositioned. This means that the adverse effect of any non-linearity ofthe PES relative to radial position of the head over the track islargely obviated. This provides the advantage that individual heads donot have to be characterised to the individual track before testing.Thus, the method may comprise testing the head without characterisingthe head to the servo track. Furthermore, this embodiment provides theadvantage that it is not necessary to re-find the track centrelinecontinuously during test before commanding the head to each position.Drift can be eliminated in real-time, enabling a greater proportion oftest time to be spent acquiring test data rather than frequentlysearching for and relocating the head over the track centreline. Thetime taken to test the head is therefore reduced, leading to moreefficient, cost-effective testing.

According to a sixth aspect of the present invention, there is providedapparatus for testing a read/write head, the apparatus comprising: aspindle for mounting to and rotating a disk medium; a positioner forpositioning a read/write head over a radial position on a said diskmedium; a controller constructed and arranged to command the positionerto position the head with a periodic movement signal such that theradial position of the head moves notionally along a periodic path inwhich the head traverses n wavelengths of the periodic path as the diskrotates m revolutions, where n and m are integers; and, a data acquirerfor taking readings from the disk using the head at a plurality ofpoints along the periodic path.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1A shows an example of a prior art QAM (quadrature amplitudemodulated) servo burst, and FIG. 1B shows an example of a PES (positionerror signal) derived from the servo burst of FIG. 1A;

FIG. 2 shows schematically an example of a read/write head testapparatus in accordance with an embodiment of the present invention;

FIG. 3 shows an example of a layout of the servo nulls of a servo trackin accordance with an embodiment of the present invention;

FIGS. 4A and 4B show examples of a spiral servo track in accordance withembodiments of the present invention;

FIG. 5 shows an example of RRO and NRRO error in reading the servo trackof FIG. 3; and,

FIG. 6A shows an example of an idealised sinusoidal commanded headtrajectory suitable for use with the test apparatus of FIG. 2, FIG. 6Bshows an example of the trajectory of FIG. 6A with noise being present,and FIG. 6C shows an example of the trajectory of FIG. 6A with thermaldrift being present.

Referring to FIG. 2, an example of a test apparatus 10 in accordancewith an embodiment of the present invention is shown. A magnetic disk 11is mounted to a spindle 12. The disk 11 has at least one circularconcentrically arranged test track 101, having servo bursts 102 arrangedin the form of a sectored servo track 109 (shown most clearly in FIG.3).

A read/write head 14 which is to be tested by the apparatus 10 ismounted to a head gimbal assembly 15. The head gimbal assembly 15 ismounted on a micro actuator 16. This is preferably a single axis microactuator 16. However, in other embodiments the micro actuator 16 can bea micro positioner in any suitable form having any number of linear orrotational axes. The micro actuator 16 is operable to make finepositional adjustments to the head 14 relative to the disk 11.

The positioning of the head 14 by the micro actuator 16 is controlled bya feedback control loop. A low frequency servo controller is preferablyused, having a bandwidth between 1 Hz and 10 Hz. It is preferred for thetest apparatus 10 to operate at relatively high rpm to reduce test time,for example about 12000 rpm (about 200 revolutions per second). As willbe described, the test apparatus 10 acquires one position error signalsample per revolution. This provides an adequate sample rate to supportthe required bandwidth.

A pre-amplifier 17 amplifies the data detected by the head 14. Thedetected data corresponding to the servo bursts 102 is passed via achannel 18 to a demodulator 19. As will be discussed in detail in thefollowing, demodulator 19 is generally operable to generate a PES 20from the servo bursts 102. This PES 20 is fed back to a servo controller21. The demodulator 19 receives a commanded input 19 a corresponding tothe desired radial offset of the head 14. This offset may be for examplerelative to the centreline of the track 101. The servo controller 21 isoperable to produce a signal output 21 a for the micro actuator 16 tocause the actuator 16 to position the head 14. The servo controller 21is generally arranged to position the head 14 to reduce the PES 20 tozero. This operation of the servo controller 21 may be accomplished byany suitable technique, such as is known in the art, and is notdiscussed in detail herein.

FIG. 3 shows (not to scale) a test track/data track 101 having apreferred layout of servo bursts 102 (shown schematically) in the formof a sectored servo track 109. The servo bursts 102 are shown over asegment of the complete test track 101, from sector n−3 to sector n+3.The track 101 is circular and concentric with the disk 11. (It should benoted that the circular track has been projected onto a linear axis inFIG. 3 to illustrate more clearly the properties of the servo track).

In use, data is written to and read from the track 101 on the disk 11interleaved with the sectored servo track 109. The servo bursts 102 ofeach servo sector provide servo nulls 108. The preferred servo bursts102 are QAM (quadrature amplitude modulated) servo bursts, as forexample shown in FIG. 1, such that each servo null 108 is provided by apair of associated servo bursts 102. However other suitable types ofservo bursts 102 known in the art for providing servo nulls may be used.For reasons of clarity, FIG. 3 does not show the form of the servobursts 102 themselves. Instead, FIG. 3 shows the location of the servonulls 108 provided by the various servo bursts 102 in relation to thetrack 101.

In a preferred embodiment, the radial positions 23 of the servo nulls108 vary linearly with the circumferential positions of the servo nulls102. FIG. 3 shows the linear relationship or locus 24 of the positionsof the servo nulls 108. As will be readily appreciated, where a sectoredservo scheme is employed, the radial positions 23 of the servo nulls 108lie on and define the locus 24 but will do so having discrete positions.Hence FIG. 3 shows the locus 24 of the discrete servo null positions 23.It is preferred that the servo nulls 108 are evenly radially spaced. Itis also preferred that the servo nulls 108 are evenly circumferentiallyspaced. In an example, if it is assumed that a typical number of servosectors is 250 and that the servo sectors take up half of the track 101,then each sector is equivalent to 0.2% of the circumference of the track101.

Other relationships between the radial positions and circumferentialpositions of the servo nulls 108 other than linear are contemplated. Inany case, preferably the radial position of successive servo nulls 108varies monotonically with circumferential position. Preferably the formof the locus 24 is predetermined and known to the system beforehand.

FIG. 3 shows that for the current position of the head 14 a null 26occurs on sector N for a concentric read position 25. As the head 14flies circumferentially over the track 101 it detects each servo burst102 in turn. The detected signal from the servo bursts 102 isdemodulated to provide a servo sample for each servo burst 102,measuring the radial position of the servo null 108 of the servo burst102 relative to the head 14.

The signal from this servo burst 102 is used to generate the errorsignal to correct for any thermal drift. To reduce the noise in thissignal, the bursts either side of the target may be read and ifnecessary averaged or interpolated using the known relationship of thelocus 24 to determine a more accurate null position. As previouslymentioned, non-linearity error and gain error are reduced near this nullposition.

Referring again to FIG. 3, if it is now desired to microjog the head 140.5% of the track width to a new radial position 27 a then the followingsteps are taken. First, a new target null position 28 is calculated forthe desired radial position 27 a of the head 14 by using the knownrelationship of the locus 24. In this example the target null position28 lies midway between sectors N+2 and N+3. The head 14 is thencommanded to the approximate target position 27 a using the microactuator 16 and its internal sensor. The servo nulls 108 are read by thehead 14. Either all servo nulls 108 can be read or only those near thetarget radial position 27 a can be read. Interpolation is used in thedetected null signals in accordance with the known relationship of thelocus 24 to find the actual null position 28 where the head crosses thelocus 24. The difference in the position of the target null position 28and the detected null position is used to generate the position errorsignal 20.

As will be appreciated, this method of calculating the target nullposition 28 for the desired radial offset of the head 14 before the head14 is moved and then generating a position error signal 20 relative tothis target null position 28 results in a position error signal 20 thatwill always be zero when the head is at the target position 27 a. Thisis true at whichever target position 27 a is selected relative to thetrack 101. This technique may be used for small microjogs across thetrack 101 or scaled to moves of several tracks 101. This technique maybe used to generate a PES for any desired radial position of the head 14relative to the track 101, so long as the locus 24 extends radially tothe same extent as the track 101. Radial positions beyond the track 101can similarly be used as long as the locus 24 extends this far.

In another embodiment the target position 27 a can be selected tocoincide with whichever is the nearest servo null 108 at that radialposition. In this case, the resolution in the positioning of the head 14is limited by the number of servo nulls 108 on the locus 24.

Preferably the servo nulls 108 extend at least across the entire radialextent of the test track 101. The servo nulls 108 may extend beyond theradial extent of the track 101. Indeed, the servo track 109 may extendbeyond the test track 101 shown in FIG. 3 to other, radially adjacenttracks 101 on the disk 11, with the servo nulls 108 continuing theirlocus 24. In this way, as shown schematically in FIG. 4A, a singlecontinuous servo track 109 may be written to the disk 11 in the form ofa spiral. Segments of this spiral servo track 109 provide in effect arespective servo track 109 a,109 b,109 c,109 d to a plurality ofadjacent concentric circular data tracks 101 a,101 b,101 c,101 d (asshown in FIG. 4A by track edges 22 a,22 b,22 c,22 d) on the disk 11.FIG. 4B shows another example where the pitch of the spiral is the widthof the track.

It is preferred that the servo bursts 102 are written with the samemicro actuator 16 that is used when positioning the head 14 bymicro-jogging during testing of the head 14. This allows the writing andreading of the servo bursts 102 to be coordinated so that anyinaccuracies in the micro actuator 16 effectively cancel out. This isgenerally in accordance with the standard method of writing servo tracks109 in a production testing environment, where a series of tracks 109 iswritten as part of an initialisation routine and remain for severalhundred or thousands of heads 14 before repeating the process.

It is preferred that repeatable errors, either written-in RRO(repeatable run out) or mechanically induced NRRO (non-repeatable runout), are compensated for dynamically in order to achieve higheraccuracy in positioning the head 14 and thus testing the head 14. TheRRO may be characterised at initialisation of the test apparatus 10, inaccordance with known techniques in the art. The mechanical error ispreferably dynamically compensated for. Preferably an algorithminterpolates each measurement on a sector-to-sector basis, based onreading PES 20 independent of radial position. Preferably the system isarranged so that at least 20 revolutions of the disk are used toaccumulate enough RRO data to prime the system before the servocontroller 21 is engaged on the PES 20 to compensate for thermal drift.It is envisaged that this will take no more than 100 ms. This could bedone in parallel with the test operation or alternatively run as a standalone operation if the overhead is acceptable for the application.

Referring to FIG. 5, the locus 24 of ideal nulls 31 and an actual set ofdetected servo nulls 30 is shown. For the detected servo nulls 30, thepoints show the RRO component of the error (for example caused byeccentricity in the spindle) and the error bars show the NRRO componentof the error (for example caused by mechanical vibration). To determinethe RRO component, detected samples 30 need to be averaged over multiplerevolutions of the disk in order to reject the NRRO component. The errorbetween each sample 30 and the ideal null 31 is characterised and keptin a dynamic RRO table, which is used to determine the position of theservo nulls 108. The repeatable component is constant for each servoburst 3 and can therefore be used when micro-jogging across tracks 101without the need to update the table.

Other arrangements of the servo bursts 102 and the track 101 areenvisaged as alternatives to the specific embodiment described abovehaving a circular concentric data track 101 and a linear/spiral locus 24of servo nulls 108. For example, it is not necessary to write acircular, concentric data track 101. The data track 101 could be writtenin the form of a spiral or sine wave, and the locus of servo nulls 108could be concentric. Other forms are possible. In each case, the locusof servo nulls 108 extends across the radial extent of the track 101.Here, radial extent of the track 101 means relative to the track 101rather than relative to the disk 11. This allows for a point to be foundon the locus 24 where the head will cross the locus 24 and detect aservo null 108 when following the track 101 at that position at allradial positions on the track 101.

It is also envisaged that the locus 24 may extend over a number ofadjacent data tracks 101, or over only a part of a data track 101,depending on the area of the disk 11 of interest according to theparticular test being performed.

In one example, the position of the head 14 is commanded to describe aperiodic path over the surface of the disk 11. A sinusoidal headtrajectory 27 b is preferred for simplicity, though in principle anyreasonable periodic path may be used. FIG. 6A show an idealisedsinusoidal head trajectory. FIG. 6B shows a sinusoidal head trajectorywith some noise present. The disk 11 has at least one track 101 that iscircular and concentric with the disk 11. The centreline of thesinusoidal path 27 b is preferably coincident with the centreline of thetrack 101. The amplitude of the sinusoid is preferably such that thehead 14 is caused to move back and forth across the entire radial extentof the track 101.

The wavelength of the sinusoid 27 b is preferably such that an integralnumber of complete wavelengths are precisely coincident with a completerevolution of the disk 11. Alternatively, a wavelength may be used wherean integral number n of wavelengths is coincident with an integralnumber m of complete revolutions of the disk 11. It will be understoodthat the key consideration here is that the head 14, after some numberof revolutions of the disk 11 (preferably just one revolution), returnsto following the same path relative to the surface of the disk 11, suchthat a repeat reading can be taken of the data under the head 14.

Other periodic paths other than sinusoidal paths are contemplated aslong as they are periodically applied to the head 14 and move the head14 continuously over at least a portion of the radial extent of thetrack 101 and position the head 14 back where it started radially beforestarting the next period.

It is necessary that the particular coincidence between the wavelengthof the commanded head position 27 b and the number of completerevolutions of the disk 11 is achieved with a high level of precision.In order to achieve the precise coincidence of the wavelength of thesinusoid and revolution of disk 11, an optical clocking technique may beused. An example of this is disclosed in our commonly assigned U.S.patent application Nos. 60/695,845 and 11/480,582 entitled “METHODS ANDAPPARATUS FOR GENERATING A CLOCK SIGNAL, FOR WRITING SERVO TRACKS, ANDFOR CHARACTERISING TIMING ERRORS IN A SERVO TRACK WRITER” filed on Jul.5, 2005 and Jul. 5, 2006 respectively, the entire contents of which arehereby incorporated by reference. Briefly, movement of at least one markthat moves synchronously with the disk 11 is detected by an opticaldetector to provide an output signal that can be processed to provide aclock signal. The mark or marks may be provided by a grating fixed tothe disk 11, or some part that rotates synchronously with disk 11, suchas the motor spindle 12 or some part of the motor itself.

Using the preferred sinusoidal head trajectory, the head 14 is made tomeasure test data across the full extent of the track 101. In prior artarrangements this had to be done by commanding the head 14 to a desiredradial offset in relation to the track 101, and making measurements overseveral revolutions of the disk 11 at this offset; the offset would thenbe changed and the process repeated until the desired profile was builtup across the radial extent of the track 101. By using the sinusoidalcommanded head position of the present example, in principle, only onerevolution of the disk 11 is required to acquire the necessary dataacross the width of the track 101. However, in practice it is preferredto use several revolutions of the disk 101 so that a greater number ofreadings can be taken for noise rejection purposes.

In the preferred scheme, where the commanded position of the head 14 isa sinusoid, the PES 20 is sampled when the position of the head 14coincides with the track centreline (i.e. when the sinusoid or otherperiodic signal is at a multiple of π radian intervals). These samplesare collected over a number of revolutions of the disk 11 and averagedor low pass filtered. The values obtained give a very low bandwidthmeasure of error caused by thermal drift (or predominantly by thermaldrift). FIG. 6C shows an example of how the commanded sinusoidalposition of the head 14 can drift off-track due to thermal drift effects(shown in exaggerated form). In a non-drifting situation, these valuesshould be zero.

This technique has the advantage of being simple to implement. Also, thePES samples 20 are obtained at the track centreline, coinciding with thepositions of the servo nulls 108 of the servo track 109 and thus theeffects of possible non-linearity and variation in gain of the PESsignal 20 are largely obviated. It is therefore not necessary tocharacterise the head 14 to the track 101 to attain an accurate measureof the thermal drift of the head 14. Drift can be detected andeliminated in real-time, enabling a greater proportion of test time tobe spent acquiring test data rather than calibrating the head 14 to thetrack 101. The time taken to test the head 14 is therefore reducedleading to more efficient, cost-effective testing.

Embodiments of the present invention have been described with particularreference to the example illustrated. However, it will be appreciatedthat variations and modifications may be made to the examples describedwithin the scope of the present invention.

For example, examples of preferred embodiments of the present inventionhave been described with reference to performing BER bathtub tests onthe read/write head of a magnetic disk. However, as the skilled personwill readily appreciate, other types of head test could be performed inaccordance with the principles of the present invention. Similarly othertypes of head for reading other types of media could be tested inaccordance with the principles the present invention.

1. A method of positioning a head relative to a disk in a testapparatus, the method comprising: commanding the head to a desiredradial position relative to a track of a disk, wherein the track has aplurality of servo bursts defining a plurality of servo nulls for thetrack, the servo nulls being positioned such that there are servo nullsat more than four different radial positions relative to the track, theservo nulls defining a predetermined locus having a known positionrelationship with the track, the locus extending across the radialextent of the track; generating a position error signal for thedifference between the actual radial position and the desired radialposition of the head by: (a) determining a target null position on thenull locus corresponding to said desired radial position of the headrelative to the track in accordance with said known positionrelationship; (b) detecting the position of at least one servo null withthe head; (c) determining from said at least one detected servo nullposition the position error of the head relative to the target nullposition; and, (d) generating a position error signal in accordance withsaid position error; and, controlling the position of the head with afeedback controller arranged to reduce the position error signalsubstantially to zero; and, writing and/or reading test data to the diskwith the head, wherein step (b) includes detecting the position of aplurality of servo nulls and step (c) includes interpolating betweensaid plurality of null positions in order to find said position error,said interpolation being calculated in accordance with the knownposition relationship.
 2. A method according to claim 1, wherein step(b) includes detecting the position of at least the servo null radiallynearest the head.
 3. A method according to claim 1, wherein the positionerror is averaged over successive revolutions of the disk to generatethe position error signal.
 4. A method according to claim 1, wherein thedesired radial position of the head is selected so as to coincide with aservo null.
 5. A method according to claim 1, wherein the track isconcentric with the disk, and the plurality of servo nulls arecircumferentially spaced and have different radial positions at eachcircumferential position.
 6. A method according to claim 1, wherein theservo nulls are evenly spaced on the disk.
 7. A method according toclaim 1, wherein the positions of the servo nulls extend, successivelyin a single radial direction on going round the track.
 8. A methodaccording to claim 1, wherein the radial positions of the servo nullsvary linearly with circumferential position.
 9. A method according toclaim 1, wherein the positions of the servo nulls define at least partof at least one spiral of servo nulls on the disk.
 10. A methodaccording to claim 9, wherein the pitch of the spiral is the width ofthe track.
 11. A method according to claim 1, wherein the servo nulllocus is concentric with the disk.
 12. A method according to claim 11,wherein the track defines a sine wave.
 13. A method according to claim12, wherein the sine wave has a wavelength equal to the trackcircumference.
 14. A method according to claim 1, wherein the disk isinitially free of servo bursts, the method comprising performing, beforestep (a), the step of writing said servo bursts to the disk.
 15. Amethod of testing a read/write head, the method comprising: moving thehead to a desired position relative to a track of a disk according tothe method of claim 1; and, testing the head.
 16. Apparatus for testinga read/write head, the apparatus comprising: a disk having a track,wherein the track has a plurality of servo bursts defining a pluralityof servo nulls for the track, the servo nulls being positioned such thatthere are servo nulls at more than four different radial positionsrelative to the track, the servo nulls defining a predetermined locushaving a known position relationship with the track, the locus extendingacross the radial extent of the track; a positioner for positioning asaid head over a radial position on the disk; a processor arranged to:(a) receive a desired radial head position relative to the track; (b)determine a target null position on the null locus corresponding to saiddesired radial position of the head relative to the track in accordancewith said known position relationship; (c) detect the position of atleast one servo null with the head; (d) determine from at least onedetected servo null position the position error of the head relative tothe target null position; and, (e) generate a position error signal inaccordance with the position error; and, a feedback controller arrangedto receive said position error signal as a feedback input, and to causesaid positioner to position said head so as to reduce said positionerror signal to zero, wherein the processor is arranged in step (b) todetect the position of a plurality of servo nulls, and in step (c) tointerpolate between said plurality of null positions in order to findsaid position error, said interpolation being calculated in accordancewith the known position relationship.